Observations on the net carbon exchange of forests in the European Mediterranean region, measured recently by the eddy covariance method, have revived interest in a phenomenon first characterized on agricultural and forest soils in East Africa in the 1950s and 1960s by H. F. Birch and now often referred to as the "Birch effect." When soils become dry during summer because of lack of rain, as is common in regions with Mediterranean climate, or are dried in the laboratory in controlled conditions, and are then rewetted by precipitation or irrigation, there is a burst of decomposition, mineralization and release of inorganic nitrogen and CO(2). In forests in Mediterranean climates in southern Europe, this effect has been observed with eddy covariance techniques and soil respiration chambers at the stand and small plot scales, respectively. Following the early work of Birch, laboratory incubations of soils at controlled temperatures and water contents have been used to characterize CO(2) release following the rewetting of dry soils. A simple empirical model based on laboratory incubations demonstrates that the amount of carbon mineralized over one year can be predicted from soil temperature and precipitation regime, provided that carbon lost as CO(2) is taken into account. We show that the amount of carbon returned to the atmosphere following soil rewetting can reduce significantly the annual net carbon gain by Mediterranean forests.

We examined the relationship between carbon mineralization (C min ), moisture and temperature in a Mediterranean forest soil under controlled and field conditions. We studied the following. 1 The temperature sensitivity at three soil depths: soil samples were incubated at 4, 10, 20 and 30 C at optimal water content of 60% of water holding capacity (WHC). Values of C min of the top layer were more than 12 times faster than those measured in deeper layers. We found a temperature sensitivity factor (Q 10 ) of 3.3, 2.7 and 2.2 for the 0-5 cm, 5-10 cm and 10-20 cm layers, respectively. 2 The relationship between C min , moisture and temperature (top layer). The sensitivity of C min to fluctuating moisture depended on temperature. However, the Q 10 was not significantly affected by soil moisture. We fitted a multiple polynomial model that predicted C min as a multiplicative function of temperature and moisture (R 2 > 0.99).3 The response of C min of soil to rewetting after 1 and 24 hours. In all cases, the response was rapid. The soil incubated at 60% WHC or less responded positively to a sudden increase in water content, with the largest increase in the 20% WHC treatment. The model predicted C min in the field well when rewetting effects were taken into account (R 2 > 0.81). These results indicate that sudden changes in soil moisture can lead to increased carbon mineralization during the dry summer. It is necessary to include such responses in models as they may represent a substantial loss of carbon in the overall carbon balance of Mediterranean ecosystems.

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